The development of disease modifying agents in Parkinson's disease has rapidly expanded the need for in vivo markers for diagnosis and monitoring disease progression. Dopamine transporter (DAT) imaging offers the promise of an objective measure of dopaminergic degeneration allowing for identification of changes in the brain that occur early in the illness, prior to clinical diagnosis. The primary goal of this project is to examine the sensitivity and specificity of DAT imaging using [123I]beta-CIT and SPECT imaging as a diagnostic marker in subjects with suspected PD or PS. We have successfully completed the Phase I pilot study for this project and have utilized these data and experience in designing this Phase II SBIR proposed protocol. The overall study design is to recruit subjects with suspected PD from participating community neurologists and compare the baseline diagnoses of the community neurologists, movement disorders experts and dopamine transporter imaging to a 'gold standard' clinical diagnosis assigned by a movement disorder expert at 12 months follow-up. The DAT imaging diagnosis will be compared to the 'gold standard' clinical diagnosis to determine the sensitivity of [123I]beta-CIT and SPECT imaging as a diagnostic marker in PD and PS. This project is a crucial step to begin to establish [123I]beta-CIT and SPECT imaging as an objective diagnostic biomarker prior to a definitive diagnosis in patients with early parkinsonian symptoms.